12.1. Haskell 98 vs. Glasgow Haskell: language non-compliance

This section lists Glasgow Haskell infelicities in its
implementation of Haskell 98. See also the “when things
go wrong” section (Chapter 9, What to do when something goes wrong) for information
about crashes, space leaks, and other undesirable phenomena.

The limitations here are listed in Haskell Report order
(roughly).

12.1.1. Divergence from Haskell 98

12.1.1.1. Lexical syntax

The Haskell report specifies that programs may be
written using Unicode. GHC only accepts the ISO-8859-1
character set at the moment.

Certain lexical rules regarding qualified identifiers
are slightly different in GHC compared to the Haskell
report. When you have
module.reservedop,
such as M.\, GHC will interpret it as a
single qualified operator rather than the two lexemes
M and .\.

12.1.1.2. Context-free syntax

GHC is a little less strict about the layout rule when used
in do expressions. Specifically, the
restriction that "a nested context must be indented further to
the right than the enclosing context" is relaxed to allow the
nested context to be at the same level as the enclosing context,
if the enclosing context is a do
expression.

GHC doesn't do fixity resolution in expressions during
parsing. For example, according to the Haskell report, the
following expression is legal Haskell:

let x = 42 in x == 42 == True

and parses as:

(let x = 42 in x == 42) == True

because according to the report, the let
expression “extends as far to the right as
possible”. Since it can't extend past the second
equals sign without causing a parse error
(== is non-fix), the
let-expression must terminate there. GHC
simply gobbles up the whole expression, parsing like this:

12.1.1.6. Numbers, basic types, and built-in classes

GHC's implementation of array takes the value of an
array slot from the last (index,value) pair in the list, and does no
checking for duplicates. The reason for this is efficiency, pure and simple.

12.1.1.7. In Prelude support

Arbitrary-sized tuples

Tuples are currently limited to size 100. HOWEVER:
standard instances for tuples (Eq,
Ord, Bounded,
IxRead, and
Show) are available
only up to 16-tuples.

This limitation is easily subvertible, so please ask
if you get stuck on it.

Reading integers

GHC's implementation of the
Read class for integral types accepts
hexadecimal and octal literals (the code in the Haskell
98 report doesn't). So, for example,

read "0xf00" :: Int

works in GHC.

A possible reason for this is that readLitChar accepts hex and
octal escapes, so it seems inconsistent not to do so for integers too.

isAlpha

The Haskell 98 definition of isAlpha
is:

isAlpha c = isUpper c || isLower c

GHC's implementation diverges from the Haskell 98
definition in the sense that Unicode alphabetic characters which
are neither upper nor lower case will still be identified as
alphabetic by isAlpha.

12.1.2. GHC's interpretation of undefined behaviour in
Haskell 98

This section documents GHC's take on various issues that are
left undefined or implementation specific in Haskell 98.

The Char type

Following the ISO-10646 standard,
maxBound :: Char in GHC is
0x10FFFF.

Sized integral types

In GHC the Int type follows the
size of an address on the host architecture; in other words
it holds 32 bits on a 32-bit machine, and 64-bits on a
64-bit machine.

Arithmetic on Int is unchecked for
overflow, so all operations on Int happen
modulo
2n
where n is the size in bits of
the Int type.

The fromIntegerfunction (and hence
also fromIntegral) is a special case when
converting to Int. The value of
fromIntegral x :: Int is given by taking
the lower n bits of (abs
x), multiplied by the sign of x
(in 2's complement n-bit
arithmetic). This behaviour was chosen so that for example
writing 0xffffffff :: Int preserves the
bit-pattern in the resulting Int.

Negative literals, such as -3, are
specified by (a careful reading of) the Haskell Report as
meaning Prelude.negate (Prelude.fromInteger 3).
So -2147483648 means negate (fromInteger 2147483648).
Since fromInteger takes the lower 32 bits of the representation,
fromInteger (2147483648::Integer), computed at type Int is
-2147483648::Int. The negate operation then
overflows, but it is unchecked, so negate (-2147483648::Int) is just
-2147483648. In short, one can write minBound::Int as
a literal with the expected meaning (but that is not in general guaranteed.

The fromIntegral function also
preserves bit-patterns when converting between the sized
integral types (Int8,
Int16, Int32,
Int64 and the unsigned
Word variants), see the modules
Data.Int and Data.Word
in the library documentation.

Unchecked float arithmetic

Operations on Float and
Double numbers are
unchecked for overflow, underflow, and
other sad occurrences. (note, however that some
architectures trap floating-point overflow and
loss-of-precision and report a floating-point exception,
probably terminating the
program).